As suggested 

"A number of Xfce applications (Xfce Terminal and Thunar for example) support the standard GTK+ way of changing shortcuts: simply hover over the menu option with the mouse pointer and press the keyboard shortcut you want to rebind it to."  

I tried to use dead_circumflex to define one of my shortcuts in xfce4-terminal. And I found that dead_circumflex is "disabled". At first, I thought this may be the bug which belongs to gtk+. However, I found I can use dead_circumflex in thunar. 

so, there should be something wrong in xfce4-terminal. 

My system::archlinux+icewm+thunar+xfce4-terminal 


Best wishes,